U.S. Pat. No. 4,656,752 to the same inventor of this application disclosed an angular level having a rotational protractor rotatably mounted in the casing, and a plumbing swing wheel pivotally secured in the protractor for measuring an angle when the level is laid on an object to be measured.
When it serves for calibrating a verticality of an object as viewed through the transparent plumbing swing wheel and the protractor, the protractor as filled with the transparent dampening oil (25a) may deflect the light and can not obtain a clear image through the lens-like protractor, thereby being impossible to provide as a plumbing instrument by directly viewing an image through the protractor.
The present inventor has found the drawbacks of the conventional angular level and invented the present optical plumbing instrument.